The present invention relates to non-destructive assay using neutron coincidence/multiplicity counting generally, but not by way of limitation, to a novel method for reduction in the interference of cosmic-ray induced neutron events and improvements in the measurement precision and accuracy.
Passive Neutron Coincidence Counting, including those systems utilizing multiplicity analysis, is commonly used to detect the presence of radio-activity in industrial samples. With the large amount of process wastes generated from operating nuclear facilities and the current interest in preparing stored materials for disposal in long term repositories such as the Waste Isolation Pilot Plant, which is the national repository for waste, the need for improvements in the performance of existing and new assay systems is expected to grow. The current neutron coincidence counting practices are limited in sensitivity due to the relatively low neutron yield from the nuclides of interest, large and variable background count rates, and interference from the non-radioactive components of the sample. These limitations result in biases in the assay result and the need for long assay times.
Methods applied to date to improve the sensitivity of neutron coincidence counting include statistical filtering (M. S. Krick and W. C. Harker xe2x80x9cMultiplicity User""s Guidexe2x80x9d LA-UR-93-1394 (1993), J. A. Mason, et.al. xe2x80x9cAbsolute Measurements of Waste using a Neutron Multiplicity Drum Monitorxe2x80x9d, 38th Annual Meeting of the International Nuclear Materials Management, Jul. 20-24, 1997), increasing detection efficiency (H. O. Menlove et.al. xe2x80x9cHENCxe2x80x9d Performance Evaluation and Plutonium Calibration, LA-13362-MS, (1997)), multiplicity counting (N. Ennslin, et. al., xe2x80x9cExpected Precision of Neutron Multiplicity Measurements of Waste Drumsxe2x80x9d, LA-UR-95-2275, Presented at the INMM 36th Annual Meeting, Jul. 9-12, 1995), and truncation of the multiplicity distribution (LANL).
The statistical filtering techniques improve the precision of the measurement by reducing, but not eliminating, the influence of fluctuations in cosmic-ray induced neutron background. The assay or count is divided into several shorter counts, based on an expected normal distribution of count rates, outliers are rejected on the assumption that the cosmic-ray induced neutron events are large and will result in a disparate count rate in the count rate for the cycle. For traditional coincidence counting these filters were applied to the total neutron count-rate, accidental coincidence rate, and the real coincidence rates. With the application of multiplicity counting to low mass samples, the filtering has been applied to the singles, doubles and triples rates. These filters have resulted in precision improvements on the order of 10 to 20% (e.g. a drop from 10% without the filters to 8% with the filters).
Since Multiplicity counting has been applied to low mass samples, attempts have been made to utilize the higher moments of the distributions, the triples and quads (M. S. Krick, xe2x80x9cThermal Neutron Multiplicity Counting of Samples with Very Low Fission Rates, Presented at the Institute of Nuclear Materials Management 38th Meeting, Jul. 20-24, 1997; M. Pickrell et.al. xe2x80x9cApplication of Neutron Multiplicity Counting to Waste Assayxe2x80x9d,), rates to improve the sensitivity and precision of the assay. It was found that the assay precision degraded by using these higher moments, although some improvement in the accuracy may have been achieved (N. Ennslin et. al. xe2x80x9cExpected Precision of Neutron Multiplicity Measurements of Waste Drumsxe2x80x9d, LA-UR-95-2275 (1995),).
An improved neutron detection system has been developed for larger sample sizes where sensitivity improvements have the greatest potential impact in terms of applications. The High Efficiency Neutron Coincidence Counter (H. O. Menlove et. al. xe2x80x9cHENCxe2x80x9d Performance Evaluation and Plutonium Calibration, LA-13362-MS, (1997)) was developed in a Cooperative Research And Development Agreement (CRADA) between LANL and Canberra to provide a large sample (200 liter drum) system with properties optimized for high sensitivity and accuracy in the assay of low mass samples. This system incorporated high detection efficiency, neutron detection short die-away time, and minimized the mass of high-Z materials in the construction of the system. The prototype system met its design objectives however, no new analysis techniques or algorithms were published or released by LANL as an end product of this CRADA.
The multiplicity distribution has recently, been utilized to improve the assay precision of low mass Cf-252 samples (M. S. Krick, xe2x80x9cThermal Neutron Multiplicity Counting of Samples with Very Low Fission Rates, Presented at the Institute of Nuclear Materials Management 38th Meeting, Jul. 20-24, 1997). It was observed that over 95% of the coincident neutrons from a non-multiplying sample would result in multiplicities of 3 neutrons or less. Events of higher multiplicity could be attributed to cosmic-ray induced neutrons events. [The cosmic-ray impacting with the substance of the counter produces a spallation event which includes many neutrons.] By rejecting any event with multiplicity of greater than 3 neutrons the effect of cosmic-ray events would be minimized. However, the reference does not discuss the limitations of this approach. For instance, fissile samples with a large fraction of non-correlated neutron events, such as plutonium flouride, would disturb the multiplicity distribution and could introduce large errors into the assay result. Additionally, this approach does not remove the cosmic-ray induced neutron events with multiplicities of less than 4 neutrons.
This analysis of the literature indicates that many of the tools needed to provide a precise and accurate assay of low fissile mass samples by neutron coincidence counting have been developed. However, a coherent protocol for integrating these techniques is required to gain the full benefit of these techniques. The analysis of the literature indicates that there are no suitable methods to fully determine the cosmic-ray induced neutron events generated within the sample or to correct the measured background rates for the moderating effects of the sample""s matrix.
Accordingly, it is a principal object of this invention to provide a method and a methodology for the complete treatment of the cosmic-ray induced neutron background and improvements in the precision of the assay by minimizing the impact of the fluctuations and magnitude of this background.
It is a further object of the invention to provide a method to separate the cosmic-ray induced neutron events generated within the sample""s matrix constituents.
It is an additional object of the invention to provide a method to correct the measured background rates for the moderating effects of the sample""s matrix constituents.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description.
The present invention achieves the above claims, among others, by providing, in a preferred embodiment, a method of enhancing the neutron coincidence assay of low fissile mass samples, by combining assay precision improvement algorithms with novel background and matrix correction algorithms, to provide lower levels of detection with higher confidence that the reported results are not subject to biases resulting from environmental or sample matrix effects. The techniques are as follows:
1. Sample Moderator Background Correction for matrix moderation effects by adaptation of the Add-A-Source Technique for correction of the fissile signal strength.
2. Known Matrix Method for Cosmic-Ray Induced Background Correction: Prior knowledge of the sample matrix is used for subtraction of the cosmic-ray induced neutron signal due to the high-Z components of the sample matrix from the neutron signal from the fissile content of the matrix.
3. Cosmic Ray Rejection Algorithms, using standard or truncated multiplicity rates corrects the cosmic-ray induced neutron component of the measured background rates without prior knowledge of the sample matrix.